Electromagnetic waves and signals (hereinafter “signals”) are utilized for many different purposes. For example, electromagnetic signals may be processed in order to convey information, such as by attenuating and/or amplifying electromagnetic wave characteristics, for instance, as is seen when modulating the amplitude, frequency or phase of an electrical current or radio frequency (RF) wave to transmit data. As another example, power may be conveyed along a wave in a controlled fashion by attenuating and or amplifying electromagnetic signals, such as is seen when modulating voltage or current in a circuit. Moreover, the uses may be combined, such as when information may be conveyed through a signal by processing power characteristics.
Electromagnetic signal processing may be accomplished through digital or analog techniques. Digital and analog attenuation and/or amplification also may be combined—that is, the same wave form may be subject to various types of digital and/or analog attenuation and/or amplification within a system in order to accomplish desired tasks.
In the processing of electronic signals, phase-locked loop systems, also known as phase-locked loops, may be used for a wide variety of purposes, such as frequency synthesizers and phase modulators in transceivers for wireless communications devices such as GSM (Global System for Mobile communications), PCS (Personal Communication System), PCN (Personal Communications Network), and DECT (Digital Enhanced Cordless Telecommunications) devices. In a typical phase-locked loop (“PLL”), a reference signal at a reference frequency is input to a phase/frequency detector along with a feedback signal derived from the output of the PLL. The output of the frequency/phase detector is filtered by a loop filter and applied to a voltage controlled oscillator to generate an output signal at the desired frequency. The output signal frequency then forms at least part of the feedback signal input to the phase/frequency detector.
A low-pass loop filter may be used in a phase-locked loop to reduce spurious signals near the baseband, as well as noise at higher frequencies. One type of loop filter for use in a phase-locked loop is a second-order or third-order low-pass filter with a single corner frequency and a constant roll-off. However, designing the appropriate filter can be difficult and involve compromise. For instance, if the corner frequency of the loop filter is set low to effectively reduce noise at higher frequencies, then it may undesirably reduce the higher frequencies of the baseband. Similarly, if the corner frequency of the loop filter is set higher to accommodate the entire baseband, then the loop filter may not effectively reduce noise at higher frequencies.
Another difficulty may arise if a phase-locked loop is used in combination with a pre-emphasis filter to provide a combined flat frequency response. If the corner frequency of the loop filter is set low to effectively reduce spurious signals close to the baseband, then it may require an undesirably high gain from the pre-emphasis filter to compensate.
Accordingly, there is a need for methods and systems for filtering electromagnetic signals in a phase-locked loop that effectively reduce both close-in spurious signals and higher-frequency noise signals without unnecessarily reducing the baseband signal or requiring a pre-emphasis filter with an undesirably high gain. There also is a need for signal modulators and transmitters that employ phase-locked loops with such filtering methods and systems.